Well if you're at 65MPH and you're rolling at, say, 2800RPM, there's a few different ways that could go:
A.) You're in 3rd gear, not in "D" (which would allow the 4th overdrive gear)
B.) You're in 4th gear, but slipping out of torque converter lockup or out of 4th altogether
C.) Your speedometer is way off and you're not actually going 65MPH
If you're not around the neighborhood of 2,000RPM to 2,200RPM at 65, the first thing I would check would be the transmission fluid condition and level when hot; these auto transmissions are very sensitive to incorrect fluid levels and will act wonky if it's not right.
Drive the truck at highway speeds for 20-30 minutes, really get it warmed up; pull back into the driveway and with the engine running, pull the transmission dipstick and check the fluid. It should be nice and red, slightly sweet smelling, and the level should be between the two lines with the word "HOT" in the middle. If it's clean but low, add more ATF through the dipstick hole. If it's black or brown, smells burnt or is otherwise gross, I would look into a drain and fill at the very least.
If the fluid checks out okay, the next thing I would get is a FSM (factory service manual, downloadable versions found on this website...) and start looking into adjusting the shift linkages or adjusting the kickdown cable. Lots of topics on both in this forum, search around if needed and you'll find what you need.
Good luck; the A440F auto transmission doesn't get the best reputation around these parts but they are tougher than they get credit for. I ran mine low on fluid, overheated it constantly and rebuilt it once, got 30ish years and probably 250K miles out of mine which would have totally killed most other auto transmissions. Now I have a 5 speed manual which I like much better, but that doesn't mean I didn't like the auto...
